The correct option is Scattering of light within the atmosphere..
Explanation
The phenomenon of the sun appearing red at sunrise and sunset is primarily caused by the scattering of light by molecules and small particles present in the Earth's atmosphere. This specific type of scattering is known as Rayleigh scattering.
Detailed Analysis:
- Composition of Sunlight: Sunlight (white light) is a mixture of seven visible colors (VIBGYOR), each having a different wavelength. Violet and blue have shorter wavelengths, while red and orange have longer wavelengths.
- Mechanism of Scattering: According to Rayleigh's law of scattering, the intensity of scattered light is inversely proportional to the fourth power of its wavelength ($I \propto \frac{1}{\lambda^4}$). This means shorter wavelengths (blue/violet) are scattered much more strongly than longer wavelengths (red).
- Effect of Path Length:
- At Noon: The sun is overhead, and light travels a relatively shorter distance through the atmosphere. Only a small amount of blue light is scattered, making the sun appear white or yellowish, and the sky blue.
- At Sunrise and Sunset: The sun is near the horizon, and light rays must travel a much longer path through the Earth's atmosphere to reach the observer. During this long passage, most of the shorter wavelengths (blue and violet) are scattered away from the line of sight. Consequently, only the longer wavelengths (red and orange) penetrate the atmosphere and reach the observer's eyes.
Evaluation of Other Options:
- Absorption of light by ozone. Absorption by ozone: Ozone primarily absorbs ultraviolet (UV) radiation in the stratosphere. It does not selectively absorb visible light to produce a red appearance.
- Reflection of light by clouds. Reflection by clouds: Clouds reflect and scatter all wavelengths of visible light almost equally (Mie scattering), which is why they appear white or grey. They do not cause the source (the sun) to appear red, although they may reflect the red light already filtered by the atmosphere.
- Refraction of light by water droplets. Refraction: Atmospheric refraction is responsible for the sun being visible slightly before actual sunrise and after actual sunset, as well as the apparent flattening of the sun's disc. However, it is not the primary cause of the red coloration.
Key Takeaway:
The red colour of the rising and setting sun is due to Rayleigh scattering. The longer path of light through the atmosphere at the horizon causes shorter wavelengths (blue) to scatter away, leaving longer wavelengths (red) to reach the observer.